1. Field of the Invention
The present invention generally relates to catalyst characterization and, more particularly, to determining the large metal particle fraction in a heterogeneous catalyst.
2. Description of the Related Art
The performance of catalysts is highly dependent on their physical and chemical properties. However, it is often difficult to directly measure physical and chemical properties of supported metal particles in catalysts, especially catalysts whose average metal particle sizes are less than 100 nm, hereafter called “nanocatalysts.” Catalyst developers therefore rely on sophisticated characterization techniques to determine the physical and chemical properties and performance characteristics of new catalyst designs.
Dispersion and metal particle size distribution influence the performance of catalysts. Dispersion D is defined according to D=NTotS/NTot, where NTotS is total amount of metal surface atoms and NTot is all metal atoms in the catalyst. Dispersion is an important property in catalysis because only atoms that are exposed to the surface are able to play a role in catalytic surface reactions. Catalyst metal particle size is also an important property because larger metal particles have less percentage of their atoms at the surface. As a consequence, a catalyst with smaller metal particles will usually outperform an equivalent amount of catalyst with identical metal concentration, but having larger metal particles.
When a catalyst contains a combination of a large metal particle fraction and a small metal particle fraction, conventional characterization methods for determining catalyst dispersion and metal particle size may not work very well to adequately describe the properties of all metal particles in the sample. A catalyst can contain two distinct metal particle fractions for a variety of reasons. They may result from a particular synthesis method that was applied and/or how the initial distribution of metal particles on the support material is changed upon aging (e.g., hydro-thermal treatment, exposure to poisons, etc.). It would be possible to quantify the large metal particle fraction by imaging all of the metal particles in a catalyst sample and measuring the size of each one, but this process would be too slow and too expensive to be applied in practice.